The present invention relates to a clamping system and, more particularly, to a system for clamping a power-dissipating device, having an electrically-active and/or thermally-active baseplate to a heatsink structure.
Many power-dissipating devices are provided in packages of a standard style, e.g., type TO-220, TO-218 or TO-247 packages. These devices have packaged high power devices (with or without integrated circuit driver chips) which are disposed on, and in electrical communication with an electrically-active and thermally-active baseplate. This conductive baseplate often has a portion extending out from the die/chip-package which portion has a hole therethrough for securing the device to a heatsink structure. Theoretically, as the securing-screw is tightened, the conductive baseplate will be pulled into intimate contact with the heatsink; however, in practice, as the screw is tightened, the conductive baseplate goes through some deformation whereby a portion of the baseplate is raised away from the heatsink. This separation results in a significant reduction in reliability and increases the resistance of heat transfer from the conductive baseplate to the heatsink, necessitating an enlargement of the actual heatsink in comparison with the theoretical requirement.
This problem is exemplified in FIG. 1, where a package 10 has a metal strip 2 supporting a semiconductor chip 6; where the chip 6 and a portion 2a of the metal strip 2 are encased in a resin housing 4. The exposed strip-portion 2b has an aperture 12, for the passage of a screw 14 securing package 10 to a heatsink 18. Screw 14 is in intimate contact with conductive strip 2 and heatsink 18, thereby electrically connecting the two. As screw 14 is tightened, strip portion 2b near screw 14 contacts the heatsink, but the strip-portion 2a distant from screw, i.e., under housing 4, may (by deformation of the strip and like mechanisms) be elevated out of contact with heatsink 18. When a portion of the package 10 lifts away from the heatsink 18 a significant reduction in heat transfer occurs, leading to a reduction in device reliability due to the increased device junction temperatures.
One prior art solution to the thermal contact problem, taught in accordance with U.S. Pat. No. 4,259,685, entitled "Clamp for Securing An Encased Power Frame to a Heat Sink", is shown in FIG. 2, wherein like reference designations are used for like elements. As shown, a clamp 20 is used to maintain intimate contact between device 10 (including the portion of a conductive strip 2, within resin housing 4, where integrated chip 6 resides) and the heatsink 18. Strip 2 supports chip 6 and has a portion 2b extending beyond housing 4 and having an aperture 12 for facilitating device 10 to be secured to heatsink 18 with screw 14. Clamp 20 has a portion 22 spaced over the exposed strip-portion 2b where screw 14 is placed. Portion 22 has rectangular legs 28 bent down on three sides of strip-portion 2b to contact heatsink 18. The legs 28 define a space which is substantially closed on three sides and is open on the fourth side toward device housing 4. At this fourth side, clamp 20 extends over device 10. Screw 14 passes through an aperture 22a of portion 22 and extends through hole 12 in conductive strip 2. Screw 14 is in intimate contact with portion 22 and heatsink 18, and the metal-legs 28 of base 22 are also in intimate contact with heatsink 18. Therefore, screw 14, base 22, legs 28 and heatsink 12 are in thermal and electrical contact. Moreover, due to the alignment of the screw with the hole within the conductive strip, there is only one configuration in which to place a clamp in relation to the device.
Other difficulties arise when the devices being attached to the heatsink are at some non-ground potential. Since in the standard packages (TO-220, TO-218 or TO-247), the semiconductor device is in electrical contact with its conductive supporting baseplate, if several devices are to be mounted to a common heatsink, and the electrical potential of the devices vary, then each device must be electrically insulated from the heatsink to avoid one device from influencing the operation of another device. However, simultaneous with this electrical insulation, adequate thermal contact must be maintained for proper operation of each device.
Therefore, it is desired to provide a clamping system for flexibly securing power supply devices to a heatsink while maintaining thermal conductivity, and providing electrical insulation, between the device and the heatsink.